The present invention relates to data storage systems, and more particularly, this invention relates to detecting damage to magnetic sensors.
In magnetic storage systems, data is read from and written onto magnetic recording media utilizing magnetic transducers. Data is written on the magnetic recording media by moving a magnetic recording transducer to a position over the media where the data is to be stored. The magnetic recording transducer then generates a magnetic field, which encodes the data into the magnetic media. Data is read from the media by similarly positioning the magnetic read transducer and then sensing the magnetic field of the magnetic media. Read and write operations may be independently synchronized with the movement of the media to ensure that the data can be read from and written to the desired location on the media.
Magnetic sensors, such as GMR sensors, are used extensively in the tape and hard disk industry. These sensors contain magnetic materials whose combined effect is to have a resistance change when subjected to a magnetic field. When subjected to low-level electrical overstress (EOS) or electrostatic discharge (ESD) current/voltage pulses, GMR sensors can be damaged. Also, corrosion can damage magnetic sensors over time, reducing their signal strength and possibly leading to failure. While GMR sensors have many metal layers, the resistance change from magnetic fields is associated with three layers: the free layer, the spacer layer, and a pinned layer. The orientation of the magnetization is fixed by an antiferromagnet (AFM) which is either a natural AFM or a synthetic antiferromagnet (SAFM), which is adjacent to the pinned layer. The resistance of the GMR sensor is determined by the vector dot product of the magnetization within the pinned layer and the free layer. One form of damage is when the magnetization of the pinned layer is reversed, which is associated with a change in the in the set of films comprising the (S)AFM. The reversal or flip of the pinned layer magnetizations can occur when a sufficiently large current pulse, possibly from EOS or ESD, whose induced magnetic field opposes the magnetization in the (S)AFM and which heats the (S)AFM above it's blocking temperature. The result of the flip in the magnetizations of the films comprising the (S)AFM is a reverse magnetized pinned layer in the GMR sensor whose amplitude may be acceptable, but whose asymmetry is far from zero. In the reverse magnetized state, the sensor often will not function properly in a drive. Other EOS or ESD damage to the sensor can alter the magnetization orientation within the pinned layer or the (S)AFM which will alter the response of the GMR sensor to magnetic fields, either internally generated by current flow or externally generated.
Published approaches used to detect the flip in the magnetizations of the pinned layer or the (S)AFM involve removing the sensor from the drive and performing a tedious transfer curve utilizing an external magnet. This procedure is time consuming and costly.
Another EOS/ESD damage which occurs is complete damage of the sensor associated with interdiffusion of the metals in the multilayer GMR stack. Because some of the layers in the GMR sensor are very thin, the sensor resistance may hardly change despite the complete damage of the sensor's magnetic properties. Higher voltage or current ESD or EOS pulses will result in significant resistance changes, including complete melting of the sensor. Because resistance values for sensors used for hard disk or tape head storage products span a range as high as a factor of two, detecting ESD or EOS damage from pure resistance measurements is difficult, if not impossible.
Even in cases where diode protection against ESD or EOS events is used, a concern still exists that sufficiently high current pulses will damage heads. It is even possible that diode protection will result in protecting heads from severe ESD or EOS damage while at the same time resulting in some of the lower level ESD or EOS damages such as magnetic damage becoming more prevalent than ESD or EOS damage with high resistance changes. A diode functions by shunting current in parallel with the sensor. The fraction of the total current which passes through the sensor is the ratio of the diode “on-resistance” divided by the sum of the diode on-resistance and the sensor resistance. Ultimately, at a high enough current, the sensor will be damaged. With diode protection, there is a potential of shifting the higher level damage such as melting of the sensor or interdiffusion of metals to lower level damage associated with magnetic damage such as the creation of magnetic domains or the flipping of the polarity of the (S)AFM or pinned layer magnetizations, so the more difficult to detect magnetic damage is more prevalent than the severe, and more easily detectable, physical damage. Thus, a method to detect magnetic damage from ESD events is needed, even when diode protection is utilized.
Another damage to GMR sensors is corrosion. The corrosion of the sensor may only affect a thin layer of the sensor perpendicular to the plane of the metal stacks (stripe). In the case of mild corrosion, the resistance of the sensor may increase substantially (5 to 10% or more) due to the conversion of one or more of the metal films being oxidized in a thin layer of the sensor (5 to 10% or more of into the depth of the sensor stripe) while not affecting the deeper portion of the metal layers. The unaffected metal layers may then function normally as a GMR sensor so that the loss in the GMR response of the sensor is confined only to the surface of the sensor near the tape bearing surface (TBS). Thus, corrosion may be associated with a significant decrease in the resistance of the GMR sensor (5 to 10% or more) while still maintaining a GMR response to a magnetic field which is comparable to the unaffected GMR sensor.